1. Field of the Invention
The present invention relates to a process for producing a vinyl chloride homopolymer or vinyl chloride-based copolymer (hereinafter generically referred to as "vinyl chloride-based polymer") by polymerizing vinyl chloride or a vinyl chloride-containing monomeric mixture in an aqueous medium while circulating the reaction mixture through a heat exchanger which is arranged in the exterior of a polymerization vessel.
2. Description of the Prior Art
Heretofore, suspension polymerization of vinyl chloride or a vinyl chloride-containing monomeric mixture (hereinafter referred to as "vinyl chloride or the like") in an aqueous medium has been carried out by charging water, monomer, polymerization initiator, dispersing agent and, optionally, other additives as required, into a polymerization vessel equipped with a jacket and a reflux condenser, and passing cooling water through the jacket and reflux condenser to remove heat of polymerization reaction, thereby controlling the reaction system at a predetermined temperature. In order to attain higher productivity, there has been a tendency toward the use of larger-capacity polymerization vessels.
In recent years, it has come to be desired to shorten polymerization time while using a large-capacity polymerization vessel, in order to achieve a further enhanced productivity. Where the capacity of polymerization vessel is increased, however, the heat transfer surface area per unit volume of polymerization reaction mixture is decreased, leading to deficiency in heat-removing capability. This problem becomes particularly conspicuous where a polymerization vessel with an internal capacity of 40 m.sup.3 or more is used.
For coping with the above problem, a method wherein an aqueous suspension mixture under polymerization is passed through a heat exchanger arranged outside of a polymerization vessel, to be thereby cooled, and is then returned into the polymerization vessel has come to draw attention as an extremely efficient means of increasing the heat-removing capability, since the method offers a remarkable gain in the heat-transmitting surface area with which the aqueous suspension mixture comes into contact to be thereby cooled (Refer to Japanese Pre-examination Patent Publication (KOKAI) Nos. 54-024991 (1979), 56-047410 (1981), and 58-032606 (1983)).
According to the above method, however, it is necessary that the circulation of the aqueous dispersion mixture through the heat exchanger should be started after a polymerization degree of about 20% is reached or exceeded or that a special type of heat exchanger should be used. Otherwise, polymer scale deposition would occur in the circulation piping, and it would be difficult to maintain good quality of the resulting polymeric product in regard of plasticizer absorption, particle size distribution, fish eyes, etc. Therefore, the above-mentioned method has not been employed industrially (R. H. Bargess, "Manufacture and Processing of PVC", Applied Science Publishers, Ltd., London, 1982, p.32).